Aromatic carboxylic acids such as benzoic, phthalic, terephthalic, isophthalic, trimellitic, pyromellitic, trimesic and naphthalene dicarboxylic acids are important intermediates for many chemical and polymer products. Aromatic carboxylic acids can be produced by liquid phase oxidation of an appropriate aromatic hydrocarbon feedstock. For example, U.S. Pat. No. 2,833,816, hereby incorporated by reference, discloses the liquid phase oxidation of xylene isomers into corresponding benzene dicarboxylic acids in the presence of bromine using a catalyst having cobalt and manganese components. As further example, U.S. Pat. No. 5,103,933, incorporated by reference herein, discloses that liquid phase oxidation of dimethylnaphthalenes to naphthalene dicarboxylic acids can also be accomplished in the presence of bromine and a catalyst having cobalt and manganese components. Typically, aromatic carboxylic acids are purified in a subsequent process involving contacting crude aromatic carboxylic acid with a catalyst and hydrogen in a reducing environment as described, for example, in U.S. Pat. No. 3,584,039, U.S. Pat. No. 4,892,972, and U.S. Pat. No. 5,362,908.
Liquid phase oxidation of aromatic hydrocarbons to aromatic carboxylic acid is conducted using a reaction mixture comprising aromatic hydrocarbons and a solvent. Typically, the solvent comprises a C1-C8 monocarboxylic acid, for example acetic acid or benzoic acid, or mixtures thereof with water. As used herein, “aromatic hydrocarbon” means a molecule composed of carbon atoms and hydrogen atoms, and having one or more aromatic ring, for example a benzene or naphthalene ring. For purposes of this application, “aromatic hydrocarbon” includes such molecules having one or more hetero atoms such as oxygen or nitrogen atoms. Aromatic hydrocarbons suitable for liquid phase oxidation to produce aromatic carboxylic acid generally comprise an aromatic hydrocarbon having at least one substituent group that is oxidizable to a carboxylic acid group for example alkyl aromatic hydrocarbons such as dimethyl benzenes and dimethyl naphthalenes. As used herein, “aromatic carboxylic acid” means an aromatic hydrocarbon with at least one carboxylic acid group.
A catalyst is also present in the oxidation reaction mixture. Typically, the catalyst comprises a promoter, for example bromine, and at least one suitable heavy metal component. Suitable heavy metals include heavy metals with atomic weight in the range of about 23 to about 178. Examples include cobalt, manganese, vanadium, molybdenum, chromium, iron, nickel, zirconium, cerium or a lanthanide metal such as hafnium. Suitable forms of these metals include for example, acetates, hydroxides, and carbonates.
A source of molecular oxygen is also introduced into the reactor. Typically, oxygen gas is used as a source of molecular oxygen and is bubbled or otherwise mixed into the liquid phase reaction mixture. Air is generally used to supply the oxygen.
Subsequent purification processes typically include contacting a solution of the crude aromatic carboxylic acid product of the oxidation with hydrogen and a catalyst under reducing conditions. The catalyst used for such purification typically comprises one or more active hydrogenation metals such as ruthenium, rhodium, palladium, or platinum, on a suitable support, for example, carbon or titania.
Many modifications and improvements have been made to the liquid-phase oxidation process, for example: U.S. Pat. No. 6,194,607 to Jhung et al. discloses the addition of an alkali metal or alkaline earth metal to the reaction mixture in the oxidation of xylene isomers to benzene dicarboxylic acids; U.S. Pat. No. 5,112,992 to Belmonte et al. discloses the addition of hafnium to oxidation catalysts; U.S. Pat. No. 5,081,290 to Partenheimer et al. discloses manipulation of acetate concentration to control the rate of oxidation.
The oxidation reaction in the liquid phase oxidation of aromatic hydrocarbons is an exothermic reaction. Heat of reaction often is removed by boiling the liquid reaction mixture. A vapor phase is formed above the liquid body within the reaction vessel as a result. Such vapor phase typically contains a significant amount of reaction solvent. Overhead gases commonly are removed from the reaction vessel to control the reaction exotherm but such removal represents a significant solvent loss. Significant solvent loss would be undesirable and recovery of solvent from the vapor phase is advantageous. The removed overhead gases can be at least partially condensed and recycled to the reaction vessel in the form of condensate or used elsewhere in the process, downstream process steps or integrated operations.
The overhead gases removed from the oxidation reaction are typically at high pressure and contain a considerable amount of energy. The recovery of energy from oxidation reaction overhead gas significantly reduces the overall energy demand of the aromatic carboxylic acid production process. The importance of such energy recovery continues to grow as global energy demand grows and as demand for particular aromatic carboxylic acids grows. Increasing environmental and regulatory constraints on many energy production methods further raise the importance of recovering process energy.
Efforts have been made to recover energy from the high pressure overhead gases by condensing such stream and exchanging the recovered heat to produce moderate pressure steam. In such condensing operations, all or substantially all of the water in the gaseous stream entering the condenser is condensed to liquid form. U.S. Pat. Nos. 5,723,656 and 5,612,007 to Abrams, incorporated by reference herein, disclose, in part, a liquid phase oxidation process wherein oxidation overhead gas is directed to a high pressure, high efficiency separation apparatus to remove at least 95 wt. % of the solvent from the oxidation overhead gas and forming a high pressure overhead gaseous stream which is directed to a means for energy recovery.
U.S. Pat. No. 6,504,051 to Miller et al., incorporated by reference herein, discloses, in part, a liquid phase oxidation process with recovery of energy from reactor off-gas as in Abrams wherein oxidation overhead gas is directed to a water removal column from which is obtained an overhead vapor stream comprising oxygen depleted off-gas, water and minor amounts of solvent and reaction byproducts. Miller et al. disclose separating the overhead vapor stream into a first portion which can be directed to an energy recovery device and a second portion which is directed to a condenser from which the condensable components are returned to the water removal column and the remaining gas can be directed to an energy recovery device.
The energy recovery schemes disclosed by Abrams and by Miller et al. can recover a significant portion of the energy available in the high pressure gaseous overhead stream. However, a significant amount of the energy available in the high pressure gaseous overhead stream remains untapped. Past attempts to recover energy by condensing all or a portion of the high pressure gaseous overhead stream have employed complete condensation. Other attempts have relied on expansion of an uncondensed stream or of the non-condensable gas from a completely condensed stream for energy recovery. As energy demand increases overall, demand for certain aromatic carboxylic acid increases, and environmental and regulatory constraints upon energy production methods increase, the importance of recovering at least some portion of such untapped energy also increases.
There is a need, therefore, for an improved process for the recovery of energy from overhead gases produced during the liquid phase oxidation of aromatic hydrocarbons to produce aromatic carboxylic acids.